I. Field of the Invention
The present invention relates to intraframe coding and decoding equipment suitable for video signals of different qualities and for a video signal communication network which includes various types of coding devices.
II. Description of the Prior Art
Conventionally, communication networks for video signals have been mainly configured as private networks. An integrated service digital network including digital video transmission service has been recently developed. In order to achieve an economic video communication system, a system is proposed which uses an intraframe coding equipment for short and intermediate distance transmissions and uses an interframe coding equipment for long distance transmission.
From the economic viewpoint described above, the long distance transmission must be performed by a three-transmission link of intraframe, interframe and intraframe coding equipment. Furthermore, in order to prevent an increase in noise and degradation of phase characteristics, it is preferred to form series-connected digital links wherein digital signals are not converted to analog signals but are transmitted as they are in the coding and decoding process.
In the interframe coding, locked sampling (i.e., phase synchronous sampling with that of the sync signal) must be introduced to improve coding efficiency. Therefore, locked sampling must be introduced in a link of the intraframe coding equipment which is digitally connected in series with the interframe coding equipment.
On the other hand, a frequency deviation of a synchronizing (sync) signal in the video signal varies from a deviation which falls within a range of .+-.30 ppm of a signal represented by a broadcast television signal to a deviation which falls within a range of .+-.1,000 ppm of an output from a commercially available, portable video tape recorder (VTR). It is possible to perform locked sampling of the former signal. However, since the latter has a great frequency deviation, it is very difficult to generate a sampling clock synchronous with a sync signal of the VTR output by a phase locked oscillator which is generally used as a sync clock generator.
A time-base corrector and a frame synchronizer are used to decrease the frequency deviation. When such units are connected to the portable VTR, the frequency deviation can be absorbed. However, these units are very expensive and unavailable to the portable VTR, so that nonlocked sampling cannot but be introduced for a video signal (e.g., the output from the portable VTR) having a great frequency deviation.
Meanwhile, portable VTRs have recently become prevalent. Under these circumstances, short and intermediate distance transmission service must be performed by utilizing the intraframe coding equipment of nonlocked sampling, although the output signal from the portable VTR cannot be transmitted over a long distance by using the interframe coding equipment.
In the public communication network, video signals of different quality, such as broadcast television signals with a small frequency deviation and VTR output with a great frequency deviation, have to be processed and transmitted. In order to transmit video signals of different quality in a digital form, the coding system must have both locked and nonlocked sampling coding functions, and must select one of these functions in accordance with the frequency deviation of video signal.
However, since the video signal network has been conventionally constituted by a private network, coding equipment has needed to have only one of the above-mentioned functions. As a result, the constitution of coding equipment for locked sampling is entirely different from coding equipment for nonlocked sampling. Assume that an input video signal is an NTSC color TV signal. Since two color signals are frequency multiplexed on the luminance signal in the NTSC signal, a high-order predictive coding method (i.e., J. YAMAGATA, H. TAKASHIMA, N. BANDO and T. DOI, "Asynchronous Intra-frame Coding with One-Dimensional Prediction", IEEE '81, Nos. 62, 1.1-62, 1.5) is introduced to predict input signals by using values of plural picture elements within a scanning line so as to perform nonlocked sampling. However, in the case of locked sampling, a two-dimensional predictive coding method (e.g., K. SAWADA and H. KOTERA, "NTSC Color TV Composite DPCM Coding System", Review of the Electrical Communication Laboratories, Vol. 27, Nos. 11-12, Nov.-Dec., 1979) is adopted utilizing correlation between the scanning lines. These coding methods have greatly different coding algorithms. In order to use both these coding methods in a single equipment, the equipment configuration is inevitably complicated.
From the viewpoint of users, if a user wishes to have coding equipment capable of coding signals of different quality, he will use coding equipment (A-type) which has two coding functions for locked sampling and nonlocked sampling, although the equipment becomes slightly expensive. The following cases may also be expected. If another user requires only a picture quality as low as that obtained by nonlocked sampling coding for all signal sources, he uses inexpensive nonlocked sampling coding equipment (B-type). Furthermore, if still another user who does not wish to code a signal (such as the output from a portable VTR) which has a great frequency deviation, he uses coding equipment (C-type) for only locked sampling. In this manner, it is considered that various types of coding equipment are present in the video communication network. Therefore the user who uses A-type equipment must select properly one of the two coding functions in order to communicate with different type of equipments.